All transport equations (momentum, energy, species) have a source term into them. When the equations are discretized, the source term will represent a direct momentum source within the cell. When you add a momentum source, you need to select the cells or regions, which will get the specific source when the discretization occurs. This can be solved either explicitly or implicitly.
In the image below, the source terms are represented as S,phi (in the general form), or Sx,Sy,Sz for the specific form.
I hope this helps,
J.A. Gutiérrez.